1. Field of the Invention
The present invention relates to an surface acoustic wave filter, a balanced type filter and a communication device.
2. Related Art of the Invention
Electromechanical functional parts using surface acoustic waves (SAW), of which wave acoustic velocity is several kilometers per second and which have characteristics such that wave energy is concentrated on the surface of a propagation medium, have received attention with the general trend toward densification of hardware, and gone into actual use in delay lines for radars and band-pass filters for television image receptors in association with development of inter digital transducer electrodes (IDT electrodes) and progress in thin film formation technology and surface treatment technology, and are now widely used as RF and IF stage filters for transmit-receive circuits of communication apparatuses.
In recent years, balance characteristics of semiconductor parts such as IC has been promoted for the purpose of improving antinoise characteristics, and balance characteristics is also required in surface acoustic wave filters for use in the RF stage. Also, in recent years, it has been required that the surface acoustic wave filter should have unbalanced-balanced type terminals or balanced-balanced type terminals, due to IC placed in the pre-stage and post-stage of the surface acoustic wave filter, and the like. In addition, longitudinal mode type surface acoustic wave filters have been widely used as filters of RF stage. For such surface acoustic wave filters, the balance characteristics is one of important parameters.
(A) The conventional longitudinal mode type surface acoustic wave filter having an unbalanced-balanced type input/output terminal will be first described referring to FIGS. 12 to 13.
The configuration of the conventional longitudinal mode type surface acoustic wave filter having an unbalanced-balanced type input/output terminal is shown in FIG. 12. In FIG. 12, the surface acoustic wave filter comprises first, second and third IDT electrodes 1002, 1003 and 1004, and first and second reflector electrodes 1005 and 1006 on a piezoelectric substrate 1001. The upper electrode finger of the first IDT electrode 1002 is connected to one of balanced type terminals 1007, and the lower electrode finger of the first IDT electrode 1002 is connected to the other balanced type terminal 1008. In addition, the electrode fingers of the IDT electrodes 1003 and 1004 located on the same side are connected to an unbalanced type terminal 1009, and the electrode fingers on the other side are grounded. The above configuration makes it possible to obtain the surface acoustic wave filter having unbalanced-balanced type terminals.
As another example, the configuration of the longitudinal mode type surface acoustic wave filter having balanced-balanced type terminals is shown in FIG. 13. In FIG. 13, the surface acoustic wave filter comprises first, second and third IDT electrodes 1002, 1003 and 1004, and first and second reflector electrodes 1005 and 1006 on the piezoelectric substrate 1001. The upper electrode finger of the first IDT electrode 1002 is connected to one of balanced type terminals 1007, and the lower electrode finger of the first IDT electrode 1002 is connected to the other balanced type terminal 1008. In addition, the electrode fingers of the IDT electrodes 1003 and 1004 located on the same side are connected to a balanced type terminal 1010, and the electrode fingers of the IDT electrodes 1003 and 1004 located on the other side are connected to a balanced type terminal 1011. The above configuration makes it possible to obtain the surface acoustic wave filter having balanced-balanced type terminals.
(B) The conventional longitudinal mode type surface acoustic wave filter having unbalanced-balanced type input/output terminals will now be described referring to FIG. 27.
FIG. 27 shows a schematic diagram of the conventional longitudinal mode type surface acoustic wave filter having unbalanced-balanced type input/output terminals. In FIG. 27, the surface acoustic wave filter comprises a first-stage filter track 6 and a second-stage filter track 12 each placed on the piezoelectric substrate.
The first-stage filter track 6 comprises first, second and third IDT electrodes 1, 2 and 3, and first and second reflector electrodes 4 and 5. Also, the second-stage filter track 12 comprises fourth, fifth and sixth IDT electrodes 7, 8 and 9, and third and fourth reflector electrodes 10 and 11.
The second and third IDT electrodes 2 and 3 are located on both sides of the first IDT electrode 1, and on both side of this arrangement, the first and second reflector electrodes 4 and 5 are located. Also, the fifth and sixth IDT electrodes 8 and 9 are located on both sides of the fourth IDT electrode, and both sides of this arrangement, the third and fourth reflector electrodes 10 and 11 are located.
The first IDT electrode 1 is constituted by an upper electrode 1a located on the side opposite to the second-stage filter track 12, and a lower electrode 1b located on the side of the second-stage filter track 12.
The second IDT electrode 2 is constituted by an upper electrode 2a located on the side opposite to the second-stage filter track 12, and a lower electrode 2b located on the side of the second-stage filter track 12.
The third IDT electrode 3 is constituted by an upper electrode 3a located on the side opposite to the second-stage filter track 12, and a lower electrode 3b located on the side of the second-stage filter track 12.
The fourth IDT electrode 7 is constituted by an upper electrode 7a located on the side of the first-stage filter track 6, and a lower electrode 7b located on the side opposite to the first-stage filter track 6.
The fifth IDT electrode 8 is constituted by an upper electrode 8a located on the side of the first-stage filter track 6, and a lower electrode 8b located on the side opposite to the first-stage filter track 6.
The sixth IDT electrode 9 is constituted by an upper electrode 9a located on the side of the first-stage filter track 6, and a lower electrode 9b located on the side opposite to the first-stage filter track 6.
In this way, the IDT electrodes are each constituted by a pair of comb electrodes, namely upper and lower electrodes.
Also, the upper electrode 1a of the first IDT electrode 1 is connected to an inputting unbalanced type terminal IN of the first-stage filter track 6 provided on the side opposite to the second-stage filter track 12, and the lower electrode 1b of the first IDT electrode 1 is grounded.
The lower electrode 2b of the second IDT electrode 2 is connected to the upper electrode 8a of the fifth IDT electrode 8 by a leading electrode 32. The upper electrode 2a of the second IDT electrode 2 is grounded.
The lower electrode 3b of the third IDT electrode 3 is connected to the upper electrode 9a of the sixth IDT electrode 9 by a leading electrode 33. The upper electrode 3a of the third IDT electrode 3 is grounded.
The upper electrode 7a of the fourth IDT electrode 7 is connected to a balanced type terminal OUT1 provided on the side of the first-stage filter track 6, of a pair of outputting balanced type terminals, and the lower electrode 7b of the fourth IDT electrode 7 is connected to a balanced type terminal OUT2 provided on the side opposite to the first-stage filter track 6, of a pair of outputting balanced type terminals.
The lower electrode 8b of the fifth IDT electrode 8 and the lower electrode 9b of the sixth IDT electrode 9 are both grounded.
Operations of this conventional surface acoustic wave filter will now be described.
By inputting a signal to the unbalanced type terminal IN, an surface acoustic wave is produced in the first IDT electrode 1. Then, the surface acoustic wave is locked in by the first and second reflector electrodes 4 and 5 to produce a plurality of resonance modes. By using these resonance modes, filter characteristics can be obtained, and conversions into electrical signals are carried out in the second IDT electrode 2 and the third IDT electrode 3, respectively.
The electrical signal converted in the second IDT electrode 2 is outputted to the upper electrode 8a of the fifth IDT electrode 8 through the leading electrode 32. Also, the electrical signal converted in the third IDT electrode 3 is outputted to the upper electrode 9a of the sixth IDT electrode 9 through the leading electrode 33. At this time, by adjusting in advance the intervals between the IDT electrodes of the surface acoustic wave filter and the way of connecting electrode fingers, the phase of the electrical signal inputted to the leading electrode 32 is made opposite to that of the electrical signal inputted to the leading electrode 33.
The electrical signal inputted to the fifth IDT electrode 8 is converted into an surface acoustic wave in the fifth IDT electrode 8, and the electrical signal inputted to the sixth IDT electrode 9 is converted into an surface acoustic wave in the sixth IDT electrode 9. Then, the surface acoustic waves converted in the fifth IDT electrode 8 and the sixth IDT electrode 9 are locked in by the third and fourth reflector electrodes 10 and 11 to produce a plurality of resonance modes, respectively. By using these resonance modes, filter characteristics can be obtained, and the electrical signals are outputted from the balanced type terminals OUT1 and OUT2.
(C) The conventional longitudinal mode type surface acoustic wave filter having a balanced type input/output terminal will now be described referring to FIG. 40.
The configuration of the conventional longitudinal mode type surface acoustic wave filter having a balanced type terminal is shown in FIG. 40. In FIG. 40, the surface acoustic wave filter has a configuration similar to that of the aforementioned conventional surface acoustic wave filter (see FIG. 12), and comprises first, second and third interdigital transducer electrodes 4102, 4103 and 4104 (hereinafter referred to as IDT electrode), and first and second reflector electrodes 4105 and 4106 on a piezoelectric substrate 4101. One electrode finger of the first IDT electrode 4102 is connected to one of balanced type terminals 4107, and the other electrode finger of the first IDT electrode 4102 is connected to the other balanced type terminal 4108. Also, the electrode fingers of the second and third IDT electrodes 4103 and 4104 located on one side are connected to an unbalanced type terminal 4109, and the electrode fingers located on the other side are grounded. The above configuration makes it possible to obtain the surface acoustic wave filter having unbalanced-balanced type terminals.
However, the above described conventional surface acoustic wave filter has the following problems.
(A) For the surface acoustic wave filter of FIG. 12, there exist near the leading electrode connecting the balanced type terminal 1007 to the first IDT electrode 1002 a wiring connecting the second IDT electrode 1003 to the unbalanced type terminal 1009, and a wiring connecting the third IDT electrode 1004 to the unbalanced type terminal 1009.
On the other hand, the leading electrode connecting the balanced type terminal 1008 to the first IDT electrode 1002 is located at a greater distance from the wiring connecting the second IDT electrode 1003 to the unbalanced type terminal 1009 and the leading electrode connecting the third IDT electrode 1004 to the unbalanced type terminal 1009, than leading electrode connecting the balanced type terminal 1007 to the first IDT electrode 1002.
Therefore, the leading electrode connecting the balanced type terminal 1007 to the first IDT electrode 1002 has a larger parasitic component of high frequency existing between itself and the leading electrode connecting the unbalanced type terminal 1009 to the second IDT electrode 1003 and the third IDT electrode 1004, than the leading electrode connecting the balanced type terminal 1008 to the first IDT electrode 1002. Thus, balance characteristics will be degraded.
For the surface acoustic wave filter of FIG. 13, a leading electrode connecting the second IDT electrode 1003 to the balanced type terminal 1010 and a leading electrode connecting the third IDT electrode 1004 to the balanced type terminal 1010 exist near the leading electrode connecting the balanced type terminal 1007 to the first IDT electrode 1002, and signals substantially identical in phase are passed through these two leading electrodes. Therefore, the parasitic component of high frequency between the leading electrode connecting the balanced type terminal 1007 to the first IDT electrode 1002 and the leading electrode from the second IDT electrode 1003 is substantially identical in phase to the parasitic component of high frequency between the leading electrode connecting the balanced type terminal 1007 to the first IDT electrode 1002 and the leading electrode from the third IDT electrode 1004.
Similarly, a leading electrode connecting the second IDT electrode 1003 to the balanced type terminal 1011 and a leading electrode connecting the third IDT electrode 1004 to the balanced type terminal 1011 exist near the leading electrode connecting the balanced type terminal 1008 to the first IDT electrode 1002, and signals substantially identical in phase are passed through these two leading electrodes. Therefore, the parasitic component of high frequency between the leading electrode connecting the balanced type terminal 1008 to the first IDT electrode 1002 and the leading electrode from the second IDT electrode 1003 is substantially identical in phase to the parasitic component of high frequency between the leading electrode connecting the balanced type terminal 1008 to the first IDT electrode 1002 and the leading electrode from the third IDT electrode 1004.
Therefore, the signals outputted from the balanced type terminals 1007 and 1008 or the balanced type terminals 1010 and 1011 contain the above described parasitic components, and an unbalanced parasitic component is generated in each of the balanced type terminals, thus compromising the characteristic of the surface acoustic wave filter.
In this way, for the conventional surface acoustic wave filter (see FIGS. 12 and 13), there are cases where leading electrodes from IDT electrodes and each IDT electrodes are spatially coupling to each other to degrade balance characteristics and compromise the characteristic of the surface acoustic wave filter.
(B) Also, for the surface acoustic wave filter of FIG. 27, a leading electrode 32 connecting the lower electrode 2b of the second IDT electrode 2 to the upper electrode 8a of the fifth IDT electrode 8, and a leading electrode 33 connecting the lower electrode 3b of the third IDT electrode 3 to the upper electrode 9a of the sixth IDT electrode 9 exist near the leading electrode connecting the balanced type terminal OUT1 to the upper electrode 7a of the fourth IDT electrode 7. On the other hand, neither leading electrode 32 nor leading electrode 33 exists near the leas wiring connecting the balanced type terminal OUT2 and the lower electrode 7b of the fourth IDT electrode 7.
In this way, the leading electrode connecting the balanced type terminal OUT1 to the upper electrode 7a of the fourth IDT electrode 7 is located at a closer distance from the leading electrodes 32 and 33 than the leading electrode connecting the balanced type terminal OUT2 to the lower electrode 7b of the fourth IDT electrode 7.
The inventor therefore believes that unbalanced parasitic components exist in the leading electrode connecting the balanced type terminal OUT1 to the upper electrode 7a of the fourth IDT electrode 7 and the leading electrode connecting the balanced type terminal OUT2 to the lower electrode 7b of the IDT electrode 7, thus degrading the balance characteristics.
In this way, for the conventional surface acoustic wave filter (see FIG. 27), there are cases where leading electrodes from IDT electrodes and each IDT electrodes are spatially coupling to each other to make the parasitic component unbalanced, whereby the balance characteristics is degraded and the characteristic of the surface acoustic wave filter is compromised.
(C) Also, the surface acoustic wave filter of FIG. 40 suffers significant degradation in amplitude balance characteristic and phase balance characteristic in the passband with the value of amplitude balance characteristic being −1.2 dB to +1.0 dB and the value of phase balance characteristic being −8° to +10° as shown in FIGS. 41A to 41C. Furthermore, FIG. 41A shows the pass characteristic of the conventional 900 MHz band surface acoustic wave filter, FIG. 41B shows the amplitude balance characteristic in the passband (from 925 MHz to 960 MHz) of the conventional 900 MHz band surface acoustic wave filter, and FIG. 41C shows the phase balance characteristic in the passband of the conventional 900 MHz band surface acoustic wave filter.
Here, the amplitude balance characteristic means a difference between the signal amplitude between one of the balanced type terminals 4107 and the unbalanced type terminal 4109 and the signal amplitude between the other balanced type terminal 4108 and the unbalanced type terminal 4109, and if this value equals 0, the balance characteristic is never degraded. Also, the phase balance characteristic means a deviation from 180° of a difference between the phase of a signal between one of the balanced type terminals 4107 and the unbalanced type terminal 4109 and the phase of a signal between the other balanced type terminal 4108 and the unbalanced type terminal 4109, and if this value equals 0, the balance characteristic is never degraded.
In this way, the conventional surface acoustic wave filter (see FIG. 40) suffers degradation of the balance characteristic, one of important electric characteristics. Furthermore, detailed discussions have been rarely made regarding causes of this degradation.